1. FIELD OF THE INVENTION
The present invention relates to paging within cellular communication systems, and more particularly, to the efficient allocation of paging distribution within a cellular communication system.
2. History of the Prior Art
Cellular radio communications is, perhaps, the fastest growing field in the world-wide telecommunications industry. Its growth has been such that in recent years the capacity of existing systems has been severely stressed to serve all of the subscribers who would like to have access to the system, particularly in major metropolitan areas. Moreover, cellular radio technology is currently moving from analog based systems, in which each subscriber communications channel is allotted to a single radio channel, to digital based systems in which a plurality of subscriber channels can be assigned to each radio channel through time division multiple access (TDMA) radio technology. In TDMA radio each channel is divided into a plurality of time slots and a digitized portion of each subscriber channel is broadcast in a different time slot.
However, despite the use of digital technology, such as TDMA, to increase cellular radio system capacity, the tremendous demand for cellular radio service is placing other demands on the system. For example, communication between the radio base stations within the system and the mobile stations within the system are divided into a plurality of voice or speech channels and at least one access or control channel, which may be either analog or digital and which may have any data rate. An illustrative one of such access or control channel is referred to as the forward control channel (FOCC).
Each mobile station which is operating within a cellular communications system must be locatable when a call is received by the system which is intended for that station. A mobile station is located by broadcasting a paging signal directed to the mobile and requesting it to respond if it receives the page. When the mobile broadcasts its page response signal to the page signal it is then placed on a voice channel by the base station and the call intended for the mobile can be connected to it through that voice channel. Cellular telecommunications systems employ a control channel such as the forward control channel (FOCC) as the means by which paging signals are broadcast into the various cells of the system in order to locate a particular mobile station. Thus, the more paging within a system the more radio traffic there is on the FOCC of the system. The continued subscriber growth within cellular systems along with the continued introduction of additional functionalities within the system will undoubtedly greatly increase the paging load within each system and place an even higher demand for FOCC capacity within each system.
The capacity of the control channel, such as the FOCC, within a system may be limited because of at least two reasons. For example, the data rate over certain forward control channels is restricted to a rate on the order of 8-10 K bits per second which is a speed limitation imposed by the technology used in that implementation. Secondly, the control channel must also be utilized to transmit other messages to the mobile stations, including, for example, voice channel designations, directed retry orders, system ordered rescan signals and system overhead message trains each of which use substantial control channel capacity each time they are transmitted. Thus, it is desirable to utilize the control channel capacity in as an efficient manner as possible by paging in as localized an area as possible and still reliably locating the mobile station being sought by the system.
In conventional cellular radio systems, paging within each system is employed to serve not only its own paging needs but also the paging needs of the various cooperating exchanges which seek to locate mobile subscribers within the exchange in response to call requests within their own exchanges. Paging provides the service of attempting to locate a mobile station's whereabouts within the exchange in order to set up a call to that mobile station.
More specifically, the paging process in mobile cellular radio systems, attempts to identify the specific cell containing that mobile, as described above in connection with the paging process. During the execution of this process, the mobile switching center (MSC) searches for the mobile by sending a sequence of paging messages on the FOCC of the system and awaits a page response. Obviously, the page message muse be transmitted to all of the cell sites covering the entire service area of the system in order to ensure that the mobile is located regardless of where it might be within the system. This implies that when the use of paging capacity on all the control channels in the exchange is required, only one mobile can be paged at any given time. This limitation on cellular system capacity has been improved by the definition of "location areas" (LA's) wherein the entire service area of an exchange is divided into a plurality of different location areas. Each LA may consist of one or more individual cells within the system. Each mobile informs the system as to its specific LA identification either periodically or whenever it crosses a location area border by means of registration access within the system. The definition of LA's allows selective paging within the system thereby conserving paging capacity resources. That is, if the LA of a mobile station to be located is known then a page message for that mobile is sent only within that particular LA. As a result of this modification of the paging process, as many different mobile stations may be paged simultaneously as there as location areas within the system which greatly increases the paging capacity of the system.
In present systems, when a page remains unanswered by the mobile station which is sought, the page must be repeated. This repetition can be either within a location area previously paged or within the entire service area (SA) of the system. The present practice within cellular radio systems is to employ the paging process to handle incoming page requests on a "first come, first served" basis. Depending upon whether the location area (LA) of the requested mobile station is known or not, the amount of paging capacity allocated to serve a particular page request is the same. That is, if the LA of the mobile station is known then the first page attempt is within the LA. Otherwise, it is within the service area SA which includes all of the LA's within the exchange. If no response is received to the page, the page is repeated either within the LA itself or within the SA.
When attempting to route a call to a mobile station, the MSC must specifically know in which cell the mobile station is located. In accomplishing the task of locating the mobile, the MSC pages the mobile station in the location area where the mobile station last registered. This prevents a global or system-wide page wherein all the cells within an exchange are paged simultaneously. If the mobile station does not answer the page request in the registered location area of its last registration, only then is service area or global paging required in order to locate the mobile.
Experience with the management of cellular communication systems has shown that a high percentage of mobile stations do not answer location area paging. There are three main causes for this problem. The first is the continual existence of earlier manufactured mobile stations which are able to store four different area identification codes (AIDs) within their internal memory. Such mobile stations will not perform forced registration if they remain within the same four location areas corresponding to the four AIDs stored within the mobile station's memory. If a mobile station moves from one of these four location areas to another, a forced registration does not occur and the mobile will not answer the page, thus requiring service area paging to contact the mobile.
As previously noted, the division of a service area into location areas was deemed advantageous to alleviate capacity problems within the FOCC, especially in those situations where the number of subscribers served by one MSC had reached a level on the order of 40,000 to 50,000 mobile stations. The FOCC cannot efficiently handle all page requests in a system utilizing only system-wide paging since all the cells within the system area are simultaneously paged. The division of the system area into location areas, which include a relatively small number of cells, was created to permit a page request to be directed to a specific location area if the desired mobile station was known to be located within that specific location area.
Whenever a mobile station crosses the border between two location areas, it is required to make a registration access to the MSC by means of a bit in the overhead message of the FOCC. Crossing of the border is detected by the mobile station due to the fact that the AID in the overhead message on the FOCC from the new location area is not equal to the AID received in a prior forced registration and stored within the mobile's memory. However, if a mobile station is able to store four AIDs within its internal memory, as the older models are able to do, then when it crosses the border between location areas whose AIDs are currently stored within the memory of the mobile station, a forced registration will not occur since the AID of the new location area is the same one of the AIDs of the prior location area stored within the memory of the mobile station. Thus, the mobile station will not answer a page directed to the location area of last registration, because the MSC will not have an accurate indication of where the mobile is currently located. This will continue as long as the mobile station remains within a group of four location areas, whose AIDs are stored within the mobile's memory, which is often the case.
The second reason why mobile stations often do not respond to a location area page relates to "rescanning" by the mobile stations. To ensure optimum performance, mobile stations access the system (for the purposes of registration, call initiation, and page response, for example) on the best available control channel. For this reason, before each access, the mobile station will "rescan" and listen to all the available control channels to choose the one having the best signal strength for access. The specification of the cellular system air interface standard causes the mobile stations to rescan both before and after every access. For example, when a mobile station detects that it has crossed a location area (or service area) border, it conducts a forced registration. The mobile station rescans for the control channel with the best signal strength, performs the forced registration message exchange and then rescans again, this time without checking whether or not the mobile station has again crossed a border. After the access, the mobile station then remains tuned to the best quality control channel it found during the second "rescan." If the second control channel found by the mobile station is different from the first, because the mobile crossed another border, the mobile station will not be tuned to the control channel of the location/service area in which it last registered and, thus, become "lost" to the system. This situation is quite common especially when a mobile station is moving along a border between location areas or exchanges. The control channels of the neighboring location areas or exchanges begin to compete with each other since both their respective signals are being received by the mobile station. The two signals will fade in and out as the mobile moves along the border.
In other words, if the second control channel belongs to a location area different than the control channel found when performing the first rescan, and the mobile is now located in the first location area, service area paging will be needed to find the mobile station. If the second control channel belongs to a neighboring exchange, the mobile station will not answer to any page at all from the first exchange and will not be able to receive any calls.
A third reason why a mobile station may not respond to a location area page is related to the addition of more and more subscribers to a mobile telephony system. In metropolitan areas, where there is a high concentration of subscribers, the cell radii are becoming smaller and smaller. The same cochannel reuse frequencies are being used by cells that are getting closer and closer in order to accommodate increased traffic density. Thus, when performing an access (specifically registrations), if the access is heard by more than one cell using the same control channel, there can be inconsistencies between where the MSC (or the mobile network if the multiple access is between cells that belong to different MSCs) believes the mobile is located and the actual location of the mobile. One solution to the capacity problem which is occurring within the available frequencies and the associated FOCC control channels is that of geographically closer frequency reuse. A utilization of both location and paging area paging allows for a multiple use of the FOCC channel since multiple mobiles can be paged using the one FOCC channel.
Because of the foregoing problems inherent in existing cellular communication systems, it is desirable to introduce a system of paging utilizing paging areas, which allow paging within a greater number of cells than that of a location area without having to page in the entire service area. Paging areas serve as a middle step between location area paging and service area paging and thereby improve the capacity and the efficiency of the forward control channel. As an example, a service area can be divided into 16 location areas. If a paging area contains four location areas, the use of paging area paging reduces the load caused by a service area page by a factor of four. Service area paging can still be utilized if the mobile station does not respond to the paging area page. However, i f the paging areas are wisely defined by the system operator, a very small percentage of the calls will need service area paging.
This system of utilizing paging areas can also be implemented on a multi-exchange basis. In such a situation, paging areas may include within their borders location areas located in adjacent service areas.